1. Field of the Invention
This invention relates to busbars utilized in electrically powered cells. In particular, this invention relates to edge and internal busbars utilized in electrochromic devices. This invention also relates to edge and internal busbars that can be utilized in other electrically powered cells such as electroluminescent and photochromic devices, thin-film batteries, and other cells that use geometries similar to the electrochromic devices described herein. Further, this invention relates to control circuits and methods to control the coloration of such electrochromic devices through an intermittent application of power.
Electrochromic (EC) devices are devices in which a change in an electrical signal applied to the EC device results in a change in an optical property of the EC device. Typically, the optical property is optical transmittance, although other properties can be affected such as, for example, optical spectral distribution or polarization. Electrochromic devices can be used for many applications, such as rear view automotive mirrors, windows, sunroofs, shades or visors for automotive and mass transportation applications, architectural windows, skylights, displays, light filters and screens for light pipes, displays, and other electro-optical devices.
A variety of technologies exist for producing chromogenic members. "Chromogenic devices", as used herein, is employed as commonly known in the art. Examples of these chromogenic devices include electrochromic devices, photochromic devices, liquid crystal devices, user-controllable-photochromic devices, polymer-dispersed-liquid-crystal devices, and suspended particle devices.
For example, electrochromic devices are discussed by N. R. Lynam and A. Agrawal in "Automotive Applications of Chromogenic Materials", Large Area Chromogenics: Materials & Devices for Transmittance Control, Optical Engineering Press, Bellingham, Wash. (1989), incorporated herein by reference. Other pertinent references include N. R. Lynam, "Electrochromic Automotive Day/Night Mirrors", SAE Technical Paper Series, 87036 (1987); N. R. Lynam, "Smart Windows for Automobiles", SAE Technical Paper Series, 900419 (1990); C. M. Lampert, "Electrochromic Devices and Devices for Energy Efficient Windows", Solar Energy Materials, 11, 1-27 (1984); JP 58-20729; and U.S. Pat. Nos. 3,521,941, 3,807,832, 4,174,152, 4,338,000, 4,652,090, 4,671,619, 4,702,566, 4,712,879, 4,793,690, 4,799,768, Re. 30,835, 5,066,112, 5,073,012, 5,076,674, 5,122,647, 5,142,407, 5,148,014, 5,239,406, and 5,657,149 each incorporated herein by reference.
Electrochromic panels are also discussed by Sapers, S. P., et al. in "Monolithic Solid-State Electrochromic Coatings for Window Applications", Proceedings of the Society of Vacuum Coaters Conference (1996), incorporated herein by reference, with regard to devices of the type shown in FIG. 1E. Devices comparable to that shown in FIG. 1E, and having photovoltaic layers for self-biasing operation are also described in U.S. Pat. No. 5,377,037.
Other related references of interest include U.S. Pat. No. 5,241,411, U.K. Patent No. 2,268,595, Japanese Laid-Open Patent No. Appln. No. 63-106730, Japanese Laid-Open Patent No. Appln. No. 63-106731, and U.S. Pat. No. 5,472,643, each incorporated herein by reference. Also pertinent is International Application No. PCT/US 97/05791, incorporated herein by reference, which pertains to electrochromic devices that can vary the transmission or reflection of electromagnetic radiation by applying an electrical stimulus to an EC device. International Application No. PCT/US 97/05791 uses a selective ion transport layer in combination with an electrolyte having at least one redox active material to provide a high-performance device.
Also suitable for use in this invention are liquid crystal devices such as those described by N. Basturk and J. Grupp in "Liquid Crystal Guest-Host Devices and Their Use as Light Shutters", Large Area Chromogenics: Materials & Devices for Transmittance Control, Optical Engineering Press, Bellingham, Wash. (1989), incorporated herein by reference.
User-controllable-photochromic devices (UCPC) are discussed in U.S. Pat. No. 5,604,626, entitled "Novel Photochromic Devices", incorporated herein by reference.
Polymer-dispersed-liquid-crystal (PDLC) devices are described by N. R. Lynam and A. Agrawal, "Automotive Applications of Chromogenic Materials", Large Area Chromogenics: Materials & Devices for Transmittance Control, Optical Engineering Press, Bellingham, Wash. (1989), incorporated herein by reference.
Suspended particle devices are discussed in U.S. Pat. No. 4,164,365, incorporated herein by reference.
Examples of chromogenic devices that emit light are described in Applied Physics Letters, Vol. 71, page 1293 (1997).
Examples of chromogenic devices that can store image patterns due to a change in an optical property of a material are described in U.S. Pat. No. 5,744,267, incorporated herein by reference.
The general control of chromogenic devices is discussed in U.S. Pat. Nos. 4,793,690, 4,799,768, 5,007,718, and 5,424,898, incorporated herein by reference.
The phenomenon of prolonged coloration of chromogenic devices is discussed in U.S. Pat. Nos. 5,076,673 and 5,220,317, each incorporated herein by reference.
FIGS. 1A through 1E depict typical examples of known electrochromic devices, while FIG. 1F shows another known type of chromogenic device.
For example, FIG. 1A depicts a layered EC device which includes a substrate 101, transparent conductor 103, electrochromic (redox) medium 105, transparent conductor 103' and substrate 101'.
FIG. 1B illustrates a layered EC device which includes a substrate 101, transparent conductor 103, EC layer 107, electrolyte (redox medium) 109, transparent conductor 103' and substrate 101'.
FIG. 1C shows another layered EC device having a substrate 101, transparent conductor 103, EC layer 107, ion-selective transport layer 111, electrolyte (redox medium) 109, transparent conductor 103' and substrate 101'.
Still another such EC device is shown in FIG. 1D. This device includes a substrate 101, transparent conductor 103, EC layer 107, electrolyte 113, counterelectrode 115, transparent conductor 103' and substrate 101'.
FIG. 1E shows an EC device having a substrate 101, transparent conductor 103, EC layer 107, electrolyte (ion-conductive layer) 117, counterelectrode 115 and transparent conductor 103'.
A typical liquid crystal or PDLC device is shown in FIG. 1F. This device includes a substrate 201, transparent conductor 203, liquid crystal or PDLC medium 205, transparent conductor 203' and substrate 201'.
Since the above chromogenic devices are known to those skilled in the art, a detailed explanation of the manner of construction and operation of such devices is not necessary.
In general, it is important to distribute the voltage to an electrochromic (EC) device uniformly in order to (i) maintain the uniformity of the coloration and bleaching of the EC device during changes between such states of coloration and bleaching, (ii) to improve uniformity in such colored and bleached states, and finally (iii) to enhance the kinetics of coloration and bleaching. As the size of an EC device increases, it becomes increasingly more difficult to maintain the desirable voltage distribution uniformity because increased size typically leads to increased resistance of various components. Such increased resistance results in voltage drops and current losses that adversely affects the uniformity of voltage distribution.
In other electrical devices, a particular spatial voltage distribution profile often is desired. As the size of such devices increases, similar to the example of EC devices, it also becomes increasingly more difficult to maintain the desired spatial voltage distribution profile because of the increasing electrical resistance of various components.
An applied voltage is commonly distributed at the periphery of EC devices through the use of an edge busbar which distributes an applied voltage to a surface conductor electrode. The applied voltage causes a response in a particular responsive property of an EC device. Consequently, spatial or temporal differences in the applied voltage will cause spatial or temporal differences in the responsive property. Ohmic losses along an edge busbar can therefore critically affect the even distribution of voltage, leading to undesirable non-uniformities in the coloration and bleaching of an EC device.
Furthermore, EC devices often employ thin film transparent conductors such as indium tin oxide (ITO) and doped tin oxides (DTO) for the surface conductor electrode. Such thin film transparent conductors are also used in a wide range of applications in other areas such as displays, solar cells, and liquid crystal devices. If the magnitude of the electrical currents in such devices is large, there can be a considerable electrical potential drop across the transparent conductor. Similar to the effect of voltage variations along an edge busbar, variations of voltage in thin film transparent conductors can also lead to spatial inhomogeneities in the device behavior as well as to slower overall device kinetics. Such effects become is increasingly noticeable and problematic with increasing device area.
Another related aspect of electrochromic devices is that they are current consuming devices. Accordingly, it is advantageous for the transparent conductors, i.e., the surface electrodes, to be very conductive. For applications such as large area EC panels, where current consumption is large, it is particularly important that the transparent conductors possess high effective conductivities. Present conventional large-area EC devices fabricated from commercially available transparent conductors such as, for example, ITO and DTO, generally possess slow kinetics and often display nonuniform coloring. For example, large-area EC devices presently are often darker at the edges than in the center.
EC devices can be fabricated on one substrate as described in U.S. Pat. No. 4,712,879; J. Gordon H. Matthew et. al., Proc. of 3d Symposium on Electrochromic Materials, The Electrochemical Society, Proc. Volume 96-24, Pennington, N.J., 1997, p. 311; and Badding, M. E., et al., Proc. of 3d Symiosium on Electrochromic Materials, The Electrochemical Society, Proc. Volume 96-24, Pennington, N.J., 1997, p. 369, each incorporated herein by reference.
Electrochromic devices can also be made using two substrates as described in U.S. Pat. Nos. 4,761,061, 4,768,865, 4,902,108, 5,142,407, 5,231,531, 5,472,643, and U.S. patent application Ser. No. 09/155,601, filed Aug. 9, 1997, now U.S. Pat. No. 6,178,034, each incorporated herein by reference.
Prior art EC devices are also described in Lynam N. R., Agrawal, A., Automotive Applications of Chromogenic materials, in Large-Area Chromogenics: Materials and Devices for Transmittance Control, SPIE Optical Engineering Press, Bellingham, Wash., 1990, p. 46 and Lampert C. L., Selkowitz, S. E., Large-Area Chromogenics: Materials and Devices for Transmittance Control, SPIE Optical Engineering Press, Bellingham, Wash., 1990, p. 22, each incorporated herein by reference.
U.S. Pat. Nos. 5,202,787 and 5,151,824, each incorporated herein by reference, show the way busbars in an EC device are typically put on the substrate edges in the prior art. As shown in FIGS. 2, 3A, and 3B, taken from the referenced patents, in a commercial EC automotive mirror which features two substrates 2223 and 2224, or 3333 and 3334, the substrates are staggered slightly with respect to each other. Spring clips 2221 and 2222, or 3331 and 3332, of a conductive material such as a copper sheet or a beryllium copper coated with tin are clipped to the two staggered edges of substrates 2223 and 2224, or 3333 and 3334, in order to provide an electrical connection to substrates 2223 and 2224, or 3333 and 3334. The two substrates must be offset, or staggered, from each other in order to expose surfaces 2225 and 3335 for the attachment of clips 2221 and 2222, or 3331 and 3332. The surfaces 2225 and 3335 are minimized in order to maximize the optical throughput area of the device--that is, to maximize the overlapping area of the two substrates. Accordingly, the prior art provides electrical connections at less than one half of the perimeter of each substrate.
When a potential is applied to the wire clips 2221 and 2222, or 3331 and 3332, only a small potential drop occurs in these clips because of their high conductivity. The small potential drop can be neglected for small dimensions. However, as the dimensions of the application increases, the potential drop can become significant. Such potential drops can be a problem because the potential drop comes at a cost to the potential available to the chromogenic elements themselves. Further, significant current flow can occur at the clips and clip junctions, thereby adversely adding to the overall current load.
The resistance of a typical wire clip used in commercial automotive mirrors that are about 12 inches (30 cm) in length is about 0.2 .OMEGA.. The conductivity associated with the clip depends on the intrinsic conductivity of the material, the geometric parameters of the clip (such as thickness, width and the length of the strip), and on the relevant contact resistance.
To demonstrate the current consumption in the EC devices, a commercial EC automotive mirror was colored by applying a DC step voltage of 1.4V. The mirror initially consumed a current of 3 mA/cm.sup.2, decreasing to 0.9 mA/cm.sup.2 in the fully colored state.
According to Hichwa, B.Pl, "Large Area Electrochromics for Architectural Applications", International Conference on Coatings on Glass, Saarbrucken, Germany, October 1996, an EC window device made from all thin films on a single substrate when powered at 1.8V showed an initial current consumption of about 2 mA/cm.sup.2 which decreased to about 1 .mu.A/cm.sup.2 in the colored steady-state. By assuming that such devices can be fabricated with the above current values scaling with size while keeping similar performance characteristics, the current consumption can be calculated for an EC automotive mirror and a single substrate EC window at different dimensions.
Table 1 shows the current consumption of the devices with two different active areas: (1) 6 inches by 6 inches (15 cm.times.15 cm) and (2) 12 inches by 12 inches (30 cm.times.30 cm).
TABLE 1 Colored (steady Device Initial current state) current type/Size consumption consumption Auto Mirror, 0.7 A 0.2 A 6 inch (15 cm) Thin film 0.5 A 0.2 mA window, 6 inch (15 cm) Auto Mirror, 2.8 A 0.8 A 12 inch (30 cm) Thin film 1.9 A 0.9 mA window, 12 inch (30 cm)
Therefore, as the size of the EC device increases, the current loads that the electrodes must carry are substantially increased.
Table 2 shows the resistance characteristics of several materials and the resistance associated with a tape with a dimension of 1 meter in length, 2 mm in width and 0.1 mm in thickness. Table 2 also shows the resistance drop in these tapes when they carry 0.1, 1 and 10 A of current.
TABLE 2 Data for one m long tape with a width of 2 mm and Resist- a thickness of 0.1 mm ivity Resis- Voltage Voltage Voltage at 25.degree. C. tance drop at drop at Drop at Material (10.sup.-8.OMEGA.m) (.OMEGA.) 0.1A (v) 1A (v) 10A (v) Aluminum 2.71 0.1355 0.01355 0.1355 1.355 Copper 1.71 0.0855 0.00855 0.0855 0.855 Gold 2.21 0.1105 0.01105 0.1105 1.105 Silver 1.62 0.081 0.0081 0.081 0.81 Tungsten 5.39 0.2695 0.02695 0.2695 2.695 ITO 200 10 1 10 100 Stainless 72 3.6 0.36 3.6 36 steel type 304 Tin 11.5 0.575 0.0575 0.575 5.75 Copper/ beryllium (98/2) Indium 8 0.4 0.04 0.4 4 Nickel 7.12 0.356 0.0356 0.356 3.56 Rhodium 4.3 0.215 0.0215 0.215 2.15 Nichrome 150 7.5 0.75 7.5 75 Solder 16 0.8 0.08 0.8 8 (Pb/Sn, 67/33) Solder 25 1.25 0.125 1.25 12.5 (Sn/Ag, 95/5) Conductive expoxy 300 15 1.5 15 150 Ablebond .RTM. 8380 Silver Frit 7 0.35 0.035 0.35 3.5 (Dupont, 1991)
As seen in Table 2, several materials incur serious voltage drops across their resistance runs (for a specific geometry) for the amount of current that must be provided to the EC cell. Since EC devices are typically powered at 1 to 3 volts, the voltage drop can significantly affect the actual voltage applied to the EC material, causing increases in coloration and bleach times, and in certain devices, leading to nonuniform coloration in the steady state condition. The problem of the voltage drop, resulting from the electrode resistance, is compounded by the increase in current when the size of the device gets larger as seen previously in Table 1. This invention is particularly useful for those EC devices where the current consumption exceeds 0.1 A during either coloring or bleaching processes.
When chromogenic devices are fabricated using two coated substrates, the typical gap between the substrates is in the range of from 10 to 1000 micrometers. As the size of the devices increases, such as for a six inch by six inch (15 cm.times.15 cm) device, in order to increase the charge throughput and to distribute the charge uniformly, it is important that busbars be applied to as much of the device perimeter as possible.
One prior art approach for a rectangular device as shown in FIG. 4, is to offset substrates 3301 and 3302 simultaneously around two edges of a corner to provide two exposed L-shaped surfaces 3304. Then, busbars 3303 are attached in a conventional way.
In another prior art approach, busbars 5503 are applied on opposite edges of exposed surface pairs 5504 by employing the geometry as shown in FIG. 5 where the rectangular substrates 5501 and 5502 are pivoted from each other so that the long dimension of each rectangular substrate is parallel to the short dimension of the other rectangular substrate.
Neither approach provides busbar coverage of a substantial perimeter of a substrate. As used herein, "substantial perimeter" means more than half of the perimeter of a substrate which is covered by a continuous busbar. For larger devices such as those bigger than about 6 inches (15 cm) in width and length, it is especially desirable to put the busbars all around the device, in a manner which covers a substantial perimeter of each substrate in order to provide the applied signal to the entire chromogenic panel evenly.
Nevertheless, as described above, as the length of the busbar run increases, the resistance undesirably increases. As a result, the prior art increases the thickness of the busbar material for large devices in order to increase the conductivity/unit length of the busbar in an attempt to maintain the desirably low resistance of the busbar. However, a major problem with the use of conventional busbars such as spring clips and wires for such large chromogenic applications arises when the thickness of the busbar exceeds the typical cell gaps. Even when the thickness of the busbar does not exceed the cell gap, the geometries of the prior art busbars and their placement limit the allowable increases to conductivities. Further, there may also be a problem around substrate corners when one continuous strip of the prior art busbar clip is used.
Another method with substantial coverage is provided all around the periphery, by using thin conductors, as shown in U.S. Pat. No. 5,066,112. However, in this case, the conductor thickness is limited by the gap between the substrates and its width.
Another approach is to make the two substrates dissimilar in size so that the edges of one substrate extend from all around the perimeter of the other substrate. In this configuration, conventional wire clip busbars can be used on the larger substrate. However, it is difficult to attach conventional wire clip busbars to the smaller substrate due to the limited gap available between the two substrates. The very close geometry could cause electrical shorting of the two substrates at the conventional wire clip busbar of the smaller substrate.
As noted above, in addition to the problem of voltage drops from the edge busbar clips, if the magnitude of the electrical currents in EC devices is large, there can be a considerable electrical potential drop across the thin film transparent conductor, leading to detrimentally slower overall device kinetics and spatial inhomogeneities in the EC device behavior. Therefore, for applications where current consumption is large, and especially where the area of the EC devices is large (e.g., chromogenic panels), it is particularly important that the transparent conductors possess large effective conductivities. Large-area EC devices fabricated from commercially available transparent conductors such as, for example, indium tin oxide (ITO) and doped tin oxide (DTO) generally possess slow kinetics and often display nonuniform coloring (e.g., darker at the edges to which the busbars are connected than in the center).
Values for the sheet resistance of commercially available transparent conductors such as ITO and DTO are typically greater than about 5 .OMEGA./sq (the units are also commonly written as .OMEGA./.box-solid.) to about 15 .OMEGA./sq. Lower sheet resistances may be obtained by increasing the thickness of the transparent conductor, but this adversely affects the optical properties (e.g., increased haziness and/or diminished transmissivity) and also adds appreciably to the cost. It is desirable to form substrates which possess appreciably lower effective sheet resistance (can be less than 1 .OMEGA./sq) at a cost that is attractive for applications such as those described above.
U.S. Pat. No. 5,293,546, incorporated herein by reference, describes a method for making a display device in which one of the electrodes is preferably a metallic grid. Preferred line widths were 20 micrometers with line spacings of 500 micrometers and line heights of 0.2 to 3 micrometers. The grid is then coated by a metal oxide (e.g., 1000 .ANG. of ITO). The invention relates to displays in which high resolution processing equipment must be used for depositing the grid pattern. Thus the cost is high, particularly if large substrates such as 6 inch.times.6 inch (15 cm.times.15 cm) or bigger are required because maintaining high precision in such a fine grid pattern over increasing areas is costly. Further, since these substrates must be over-coated with ITO, they are unable to use more cost effective, mass produced transparent conductors, such as mass produced ITO or inexpensive DTO deposited onto glass sheets in a float line.
U.S. Pat. No. 4,768,865, incorporated herein by reference, describes the use of a free-standing metallic grid as one of the transparent conductors. In this invention, the metal grid participates directly in the electrochemical reaction in the EC cell. However, for most EC devices, it is not desirable for the electron conductor also to participate in the reaction.
U.S. Pat. No. 5,724,176, incorporated herein by reference, describes the use of a counterelectrode for a smart window that contains a transparent substrate and a linear electrically conductive material formed on a surface of the transparent substrate. A layer of an electrochromic material is formed on the window's surface, and a layer of an electrolyte is arranged between the counterelectrode and the electrochromic electrode and in contact with the layer of the electrochromic material. Various patterns are described for the placement of the linear electrically conductive material.
U.S. Pat. No. 5,066,111, incorporated herein by reference, describes laminated EC devices. A metal grid on a glass substrate is employed as one electrode and a longitudinal set of busbars, preferably composed of a metal foil such as copper, or an electroconductive ceramic frit deposited on glass or on the surface of an electro-conductive film, is employed as the other electrode. The electrochromic film is deposited over the second electrode. Thus, the metal foil or frit conductors of the U.S. Pat. No. 5,066,111 invention are always in direct contact with either the electrochromic coating or the electrolyte. However, such direct contact can decrease the device lifetime because of reaction between the coating and the electrolyte or electrochromic coating. Moreover, if put on glass and then coated with the transparent conductive coating (TCC), other problems can arise. Most importantly, the TCC is usually deposited in a thickness of less than 0.3 micrometers. In comparison, tapes or underlying frits, etc., are typically in thicknesses of 10 to 1000 times the thickness of TCC. Thus, vacuum methods that are typically used to coat TCC have difficulty getting a conforming coating that adequately covers the edges. The reference does not address the relationship of the busbar thickness and width to the device size.
POLYCHROMIC.TM. solid films are described in European Patent Publication No. EP 0 612 826 A1, incorporated herein by reference. The reference describes how polychromic solid films may be used in electrochromic devices, particularly glazings and mirrors, whose functional surface is substantially planar or flat or that are curved with a convex curvature, a compound curvature, a multi-radius curvature, a spherical curvature, an aspheric curvature, or combinations of such curvature.
Often, a demarcation means, such as a silk-screened or otherwise applied line of black epoxy, may be used to separate the more curved, outboard blind-spot region from the less curved, inboard region of such electrochromic mirrors. The demarcation means may also include an etching of a deletion line or an otherwise established break in the electrical continuity of the transparent conductors used in such mirrors so that either one or both regions may be individually or mutually addressed. Optionally, this deletion line may itself be colored black. Thus, the outboard, more curved region may operate independently from the inboard, less curved region to provide an electrochromic mirror that operates in a segmented arrangement. As described in European Patent Publication No. EP 0 612 826 A1, upon the introduction of an applied potential, either of such regions may color to a dimmed intermediate reflectance level, independent of the other region, or, if desired, both regions may operate together in tandem.
An insulating demarcation means, such as demarcation lines, dots and/or spots, may be placed within electrochromic devices, such as mirrors, glazings, optically attenuating contrast filters and the like, to assist in setting out the interpane distance of the device and to enhance overall performance, in particular the uniformity of coloration across large area devices. Such insulating demarcation means, constructed from, for example, epoxy coupled with glass space beads, plastic tape or die cut from plastic tape, may be placed onto the conductive surface of one or more substrates by silk-screening or other suitable technique prior to assembling the device. The insulating demarcation means may be geometrically positioned across the panel, such as in a series of parallel, uniformly spaced-apart lines, and may be clear, opaque, tinted, or colorless, and appropriate combinations thereof, so as to appeal to the automotive stylist.
As described in European Patent Publication No. EP 0 612 826 A1, a demarcation means may be used that is conductive as well, provided that it is of a smaller thickness than the interpane distance and/or a layer of an insulating material, such as a non-conductive epoxy, urethane or acrylic, is applied thereover so as to prevent conductive surfaces from contacting one another and thus short-circuiting the electrochromic assembly. Such conductive demarcation means include conductive frits, such as silver frits like the #7713 silver conductive frit available commercially from E.I. du Pont de Nemours and Co., Wilmington, Del., conductive paint or ink and/or metal films. Use of conductive demarcation means, such as a line of the #7713 silver conductive frit, having a width of about 0.0937511 (0.238 cm) and a thickness of about 50 .mu.m, placed on the conductive surface of one of the substrates of the electrochromic device may provide the added benefit of enhancing electrochromic performance by reducing busbar-to-busbar overall resistance and thus enhancing uniformity of coloration, as well as rapidity of response, particularly over large area devices. However, the non-conductive layers are applied in a way which does not prevent the underlying frit lines from making contact with the electrolyte or electrochromic layers. Thus, this frit may potentially react, especially when coloring and bleaching potentials are applied.
As described above, electrochromic (EC) devices are used to reversibly vary the light transmission or reflection by application of an electrical voltage. Applications of electrochromic devices include windows for architectural use (windows, interior partitions, skylights, light pipes), windows in transportation (automobiles, trucks, planes, trains, boats, etc.), eye-wear, and displays (including large area signage).
Electrochromic windows in buildings can provide higher energy efficiency as compared to static transmission windows, while increasing the user comfort by controlling illumination and reducing glare. The same benefits can accrue for transportation uses where the user comfort is enhanced by reducing solar heat and glare during the day, while reducing the cooling load on the air-conditioner. In many of these applications the EC device can be required to be kept in a certain desired state of transmission for long periods of time. For example, a window may be kept in a darkened or bleached state for many hours of the day and may even be kept in this state for many days.
Thus, it is desirable to enhance the durability of EC devices that are used in this long single state mode, while reducing energy consumption of the EC devices. Reducing energy consumption is particularly useful in circumstances where solely a battery is used to power such a device and thus, it is important to ensure that the battery drain is minimized. Such circumstances include use in a car, aircraft, watercraft, or eyeware. One aspect of the present patent describes circuitry which addresses one or more of these issues.
U.S. Pat. No. 5,148,014, incorporated herein by reference, describes the use of a linear regulated power supply to power an EC mirror.
U.S. Pat. No. 5,193,029, incorporated herein by reference, describes the use of a Zener diode and transistor, which is essentially a linear regulation, to provide voltage to an EC mirror.
U.S. Pat. No. 5,220,317, incorporated herein by reference, describes the use of a voltage divider consisting of series resistors to scale down the voltage provided to EC elements.
Electrochromic devices which will benefit from this invention are well known in the art. For example, these are described in U.S. patent application Ser. Nos. 09/155,601, now U.S. Pat. No. 6,178,034 and 08/699,940, now abandoned, filed Apr. 9, 1997 and Aug. 20, 1996, respectively.
For those EC devices which are colored by applying a voltage, it would be desirable not to require applying continuously the coloring potential after the required coloration depth has been reached. Such continued application of the coloring potential, while promoting EC reactions, can also promote side reactions which could have detrimental effect on the device longevity. This applies for all EC devices which need to be maintained in a state of coloration that is different from their natural transmission state. The natural transmission state of an EC device is measured at equilibrium with no applied potential and when the potential difference between the opposing electrodes is zero.
Typically, the EC devices can be kept colored for a finite period of time when the coloring potential is removed, i.e., the color of the device will change towards its natural state over a period of time. This change could occur over a wide range of time intervals, from fast over several seconds or minutes, to as slow as extending up to many days, depending on the device. A device where this change is fast is said to have short "memory" and one where the change is slow is said to possess long "memory". For example, U.S. Pat. No. 6,178,034 discloses devices with long memories and compares them with devices that have short memories.
It is clear that it would be advantageous to be able to maintain a coloration setting without having to maintain an applied voltage to electrochromic devices because circuitry that allows intermittent adjustment of the voltage as needed to maintain a coloration setting would lead to lower power consumption in the device.
U.S. Pat. No. 5,384,578 (to Lynam et.al.), incorporated herein by reference, describes the use of intermittent voltages for continuously variable mirror and windows, but does not relate to changing the voltage-on or voltage-off periods and voltage-time shapes under different conditions as discussed in the present invention.
U.S. Pat. No. 4,298,970 (to Saegusa), incorporated herein by reference, describes a technique for utilizing an intermittent technique to drive EC displays with memory. The patent only describes bimodal displays which have only two states, i.e., colored and bleached states and does not discuss devices which need continuously variable light transmission across a continuum of transmissive states.
U.S. Pat. No. 5,007,718, incorporated herein by reference, describes a method of driving electrochromic elements by using a current stabilizing circuit and a voltage stabilizing circuit in tandem with a power supply to form a stabilized power source, and applying a gradually increased coloring voltage and a gradually decreased discoloring voltage to keep the current flow within predetermined amounts.
U.S. Pat. No. 5,365,365, incorporated herein by reference, describes an electrochromic system for controlling the color state by determining the charge needed to obtain a set color from the discharge potential of the system and the coloration set-point. An integrator measures the charge passing through the system and compares it to the charge to be transferred, which is measured by a differential amplifier which compares a discharge potential measured by a capacitor with a selected color set-point.
U.S. Pat. No. 5,231,531, incorporated herein by reference, describes an electrochromic system in which a voltage generator is connected to electrically conductive films by an electrical control circuit. The voltage generator receives a set-point from a control unit and generates a potential differences as a function of the temperature of the electrolyte.